A ground fault in a rotating electrical machine connected to an electrical network may pose a considerable danger for continued operation of the machine. Whereas a single ground fault close to the neutral point may not cause any immediate danger to the machine, the occurrence of the next ground fault will generate large circulating currents that can produce severe damage. To limit ground current at a single ground fault near the terminals, the machine is often grounded via an impedance means to limit mechanical and thermal stresses, thus reducing the resultant damage to the machine. Such a grounding system is often used to provide a means for detecting ground faults within the machine.
One of the existing methods for providing protection for a ground fault is based on signal injection, wherein a test signal is injected at a frequency different from the fundamental system frequency at which a rotating electrical machine is operated. By measuring the change of measured electrical quantities, such as current and/or voltage, resulted by the injected signal, the impedance to ground can be estimated and a ground fault can be detected based on the estimated impedance. However, harmonics are produced by the machine, which, during acceleration and retardation, may coincide with the frequency of the injected signal. A coincidence results in unreliable measurement and thus an unreliable impedance estimation. Consequently, a false protective operation during start and/or stop of the machine may be conducted.
U.S. Pat. No. 5,508,620 describes a method for detecting ground faults on the conductor of an electrical machine, where a measured signal is numerically evaluated. This means that a ground fault resistance is calculated directly in an evaluation unit using the injected voltage and the measured current. However, this measurement principle is only reliable if the injection voltage is a sole source of an injected frequency. Therefore, the system has to be continuously tested to determine whether the injected signal is corrupted by evaluation also during periodic pauses in the injection.
An alternative solution is described in CN101414744, wherein injection signal distortion due to interfering signals during acceleration and deceleration of a generator is avoided by injecting test signals at two different frequencies. Firstly, the generator operation frequency is detected; when the generator operation frequency is more than 10 Hz and less than 40 Hz, the injection power frequency is taken as 100 Hz; when the generator operation frequency is not more than 100 Hz or not less than 40 Hz, the injection power frequency is taken as 20 Hz. By using the dual frequency injection, the method is able to eliminate the signal influence generated by the generator during acceleration and deceleration. However, this solution is rather complicated and requires special hardware and connections. Further, the system needs to be commissioned at both frequencies.